Well it’s not the amount which come across as frightening, it’s the speed of it. We are a country of immigration anyways, especially for European populations. For instance, we took in a large batch of former Yugoslavia refugees when war broke out there in the nineties. A part turned back to their respective newly formed countries, others stayed with us and are fully integrated as we speak.
These days, as another example, we have around 200k people originally coming from Kosovo, who are now fully integrated. That’s one fifth of the total Kosovo population, and 2,5% of ours! It hasn’t gone without problems initially, but now all is fine more or less, and, in the long term, this immigration wave has turned out as to be more positive than negative imo.
So, taking in 300k people from Ukraine, while a huge task, is achievable over time, and can work out as a positive too as you say. But it’s the potential speed of it… We simply haven’t the housing commodities to take on such a large number within a few months. According to recent articles I’ve read, we have an infrastructure currently allowing us to accept roughly 60k refugees every year. So, what are we supposed to do with the rest? Construct refugee camps somewhere in the mountains and put them in there for five years or so? That would be against everything we usually do with refugees (well, it happened during WW2, so we’ll see).
On the other hand, we can’t accept to let potentially ten millions of people in camps at the Polish-Ukrainian border, so we’ll have no choice but to do something about it, and to take in our fair share, notwithstanding the problems to house these people, to school their children, to find them work etc…
Please note that European countries, from Poland to Ireland, from Portugal to Norway will face the same problems. So, if the States can indeed take a bigger part of refugees than the measly 100k announced so far, it will be more than welcome…